1. Field of the Invention
The present invention relates to an ophthalmic lens having a progressive multifocal zone and a method of manufacturing the ophthalmic lens. More particularly, the present invention relates to a contact lens having an aspheric progressive multifocal zone produced by rotating a portion of a spiral curve about an axis.
2. Discussion of the Related Art
Many individuals suffer from a condition known as presbyopia, which requires visual correction for near, distant, and intermediate distances. Such individuals are referred to as "presbyopes." Currently, presbyopes most commonly use bifocal lenses to partially remedy their condition. Although bifocals provide visual correction for near and distant vision, bifocals do not, however, provide visual correction for intermediate distances. To this end, trifocals were developed to provide visual correction for a single intermediate distance. However, trifocals do not completely remedy presbyopia because they do not provide visual correction for all possible intermediate distances.
In an effort to overcome the deficiencies of bifocals and trifocals and to provide a complete remedy for presbyopia, lens designers have developed lenses that include an aspheric progressive multifocal zone between near and distant correcting zones in order to provide continuous visual correction for all intermediate distances. Such multifocal lenses have failed to gain wide acceptance from presbyopes for various reasons stemming from the particular designs of the multifocal lenses.
Multifocal lenses utilize one of two basic approaches to provide correction for all distances. The first approach is known as alternate vision correction in which the presbyope looks through separate correcting zones of the lens to correct for the specific distance at which a viewed object is situated. Although designers typically utilize this approach for spectacle lenses, designers may also utilize this approach for contact lenses. To develop an alternate vision correction contact lens, designers typically utilize prisms or ballasts to maintain orientation of the contact on the eye. The wearer may move the contact lens relative to the position of their pupil by means of a protruding ledge, in order to look through the region of the contact lens providing the appropriate correction for the current viewing distance.
Although bifocal lenses designed with the alternate viewing approach have been reasonably successful, multifocal lenses having more than two correcting zones and designed with this approach have not gained wide acceptance due to their reliance upon the wearer to properly reposition the lens or to tilt their head each time the viewing distance changes. Such manual manipulation of the lens is extremely problematic while driving a vehicle. Moreover, proper manual positioning of a multifocal contact lens having a progressive zone is next to impossible.
The second approach in designing multifocal lenses is known as simultaneous vision correction in which correction of vision is available for all distances without requiring repositioning of the lens relative to the pupil or tilting of one's head to look through the appropriate correction zone. Simultaneous vision correction relies upon the ability of the individual's brain to selectively choose a sharp image when sharp and blurry images are simultaneously projected on their retinas. Lens designers have exerted much effort into the design of the aspheric progressive multifocal zone, which is responsible for intermediate distance viewing correction, in order to simultaneously project images on a wearer's retinas such that a sharp image for the appropriate viewing distance can be readily selected by the wearer's brain. However, despite these efforts, lens designers have achieved only limited success in designing a multifocal lens that performs this function.
Due in part to a misunderstanding of the shape of a human eye and, in part, to previous manufacturing limitations, conventional contact lenses typically include an aspheric progressive multifocal zone that is formed by rotating a conic section about a central axis. As used herein, the term "conic section" refers to a curve that results from intersecting a cone with a plane at any angle. A conic section may be mathematically defined in terms of "eccentricity." The eccentricity e of a conic section equals the distance between a point on the conic section curve P and a focus F divided by the distance between a point on the conic section curve P and a directrix L (e =g(P,F)/d(P,L).
Conic sections may be characterized by their eccentricity. A conic section having an eccentricity equal to zero (e=0) is a circle. A conic section having an eccentricity less than one (e&lt;1) is an ellipse. A conic section having an eccentricity equal to one (e=1) is a parabola. And, a conic section having an eccentricity greater than one (e&gt;1) is a hyperbola.
Lens designers have been forming aspheric progressive multifocal zones by rotating conic sections because it has been commonly believed that the human eye may be described in terms of conic sections. Contact lenses having an aspheric progressive multifocal zone by rotating conic sections have been described in U.S. Pat. No. 4,640,595 issued to David Volk and U.S. Pat. No. 5,214,453 issued to Mario Giovanzana. In these patents, the conic sections are described as having continuously varying eccentricities. This approach illustrates the difficulties in attempting to mathematically describe the human cornea. If the eye could be described in terms of conic sections, the visual shape of the eye is hyperbolic, but readings from photo-electric keratoscopes (PEK) indicate that the shape could be described as elliptical. This contradiction implies that the cornea is either described by a much more complicated mathematical formula or is a more random shape which could only be described with Chaos mathematics. The latter implication being the most probable.
Studies have shown that the shape of the cornea changes with diameter even when central keratometer "K" readings are the same. Thus, fitting a conic section off the central K readings of an individual's corneas, is not a sound approach to contact lens design.
Prior to the conception of the numerically controlled lathe, aspheric surfaces were formed using cams. Because these cams have been expensive to produce, lens designers took the approach of designing aspheric lens surfaces which can be produced using a single cam resulting in aspheric lens surfaces having the same shape factor regardless of the individual needs of a patient. Lens designers did not take different approaches because of the expense involved in making a cam for each shape factor that may be required by a patient. Despite the present availability numerically controlled lathes, lens designers have not varied their approach, and continue to design lenses which do not meet the needs of the individual patients.
The "Add" of a multifocal lens is the difference in dioptric power between the near and distant correction zones. As discussed above, designers of conic section aspheric lenses have typically used only one shape factor and/or numerical eccentricity and have tried to maximize the Add before they loose the fitting characteristics. This approach tends to limit the obtainable Add to less than 2.0 diopters. In other words, such a lens is "Add-limited." However, a large number of presbyopes exist who require Adds that cannot be obtained with this approach.
Another reason designers of conic section aspheric lenses typically use only one numerical eccentricity or one range of eccentricities for all contact lenses is that designers previously believed that the eccentricity of the aspheric surface defining the progressive zone need not vary for different Adds. Thus, lens designers were not previously aware that there was any reason to vary the eccentricity for individuals requiring different Adds.